My wife and I both use iBook to read PDFs. May not be as full-featured as the others mentioned here but it's working well for us.
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RichardNov 6 '11 at 12:46

I highly recommend this because safari supports downloading of PDFs right into iBooks. Also i find that the rendering is great, and iBooks' built in brightness display is awesome!
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XAleXOwnZXSep 3 '12 at 20:42

I use Air Sharing which creates a virtual drive on your wireless network. I can copy files (including MS Office and PDF files) to my iPhone and view them. There are other file transfer apps that do similar things.

If you're starting with documents that are not PDFs (e.g. Word files), you might find reading PDFs inconvenient since PDFs are inherently page oriented. Stanza's FAQ page has an answer about creating ePub content from other documents.

Readdle has an iphone app that allows you to download files in many formats from the web to your iphone. They give you some free storage space at their site, but you can sync the files to your phone for reading on a plane, etc.

This application seems to be available only for jailbroken devices!
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brandstaetterSep 28 '09 at 12:56

@brandstaetter: Is this a +1 or a -1 argument?
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Tim BütheSep 28 '09 at 14:41

I would say -1 - I have not looked at it, but being free would not be enough of a positive aspect. But at least it should be pointed out that this app requires jailbreaking, which currently is not possible on the 3GS and iPod touch 2nd and 3rd gen.
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brandstaetterSep 28 '09 at 16:04

(not possible with the most recent firmware, that is)
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brandstaetterSep 28 '09 at 16:05

Another possibility is to use Calibre, it can convert PDF and other ebooks into epub and it is compatible with Stanza on the iPhone for downloading docs into the phone. Internface is definitely not up to Mac standards but it works.